A small VoIP company called iCall has ported their software to the iPhone, allowing users to reroute ongoing calls to their free VoIP service in a matter of seconds. The app actively scans for accessible Wi-Fi networks and prompts the user to transfer the call once connected.
It goes without saying that the service won't work over Edge, and will almost certainly not place calls over 3G data, but considering how much of the day many people spend drenched in Wi-Fi, iCall has some serious minute-saving potential. iCall should be available as soon as the official App Store is up and running, and will likely be just as free as the desktop client. The makers sound confident that they will be listed in the app store, which provides additional confirmation that VoIP services won't be totally barred from the platform. See below for a Starbuckian demo.